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+# Fog Substrate Position in SourceOS
+
+This document captures the **OS-substrate responsibilities** for the Fog layer.
+
+`SourceOS` is the immutable, local-first substrate for workstation and edge lanes. The Fog layer depends on this repo for **machine invariants**, not for higher-level distributed semantics.
+
+## What belongs in SourceOS
+
+### 1. Local storage substrate prerequisites
+
+Every eligible SourceOS node/workstation should be able to realize a local storage substrate suitable for ephemeral fog workloads.
+
+This includes:
+
+- LVM2 availability
+- volume group and thin-pool conventions
+- safe disk targeting rules
+- stable mount-point expectations
+
+### 2. Canonical fog directory contract
+
+The substrate should reserve a stable host-side contract such as:
+
+- `/srv/fog/projects`
+- `/srv/fog/models`
+- `/srv/fog/datasets`
+- `/srv/fog/topics`
+- `/srv/fog/vector`
+- `/srv/fog/cache`
+- `/srv/fog/logs`
+- `/srv/fog/secrets`
+- `/srv/fog/tmp`
+
+These paths are substrate-level invariants so workstation, node, and container lanes can share one filesystem grammar.
+
+### 3. Container-host baseline
+
+SourceOS should define the default local execution posture for fog-capable nodes/workstations:
+
+- Linux-first container host defaults
+- rootless-friendly execution where possible
+- mount wiring to the fog directory contract
+- no dependency on opt-in community automation
+
+### 4. TopoLVM readiness
+
+When a node participates in Kubernetes lanes, the substrate should provide the prerequisites needed for a local CSI/LVM stack such as TopoLVM.
+
+Important layering rule:
+
+- local PV provisioning belongs at the substrate/infrastructure layer
+- replication and eventual-consistency semantics belong above the substrate in topic/agent layers
+
+## What does not belong in SourceOS
+
+- topic replication policy semantics
+- Hypercore topic membership logic
+- compute marketplace settlement rules
+- opt-in automation workflows
+
+Those belong in the spec, spine, and automation layers instead.
+
+## Downstream repos expected to refine this
+
+- `SourceOS-Linux/sourceos-spec` — typed contract surface for FogVault / FogCompute
+- `SociOS-Linux/socios-ignition` — first-boot realization of disk/mount/systemd defaults
+- `SociOS-Linux/workstation-contracts` — conformance checks for the machine contract
+- `SociOS-Linux/agentos-spine` — assembled runtime/integration lane
+- `SociOS-Linux/socios` — optional signed catalogs and automation